Design Principles for Ionic Liquids in Carbon Capture Revealed by an Interpretable Chemical Force Field

Ionic liquids (ILs) are promising CO2-capture solvents, yet rational design is slowed by experimental screening and limited predictive models. We extend the INTERFACE force field (IFF) to 34 ions and CO2, reproducing densities (±1%), vaporization enthalpies (±7%), and self-diffusion coefficients (±0.2 log units) relative to experiment, validated for 10-20% of the dataset, surpassing classical and polarizable models. We find that bonding in ILs is ~85% ionic and ~15% covalent, and ionic charges of ±0.85e eliminate the need for explicit polarizability while improving transferability. Molecular dynamics (MD) screening of 29 ILs and one CO2-binding organic liquid yields quantitative design principles linking structure to volatility, sorption, diffusivity, and heat capacity. Custom visual aids for electrostatic interactions illuminate differences in local electric fields and their relation to these properties. Large ions and highly localized charge reduce volatility; anions with localized charges and numerous binding sites enhance CO2 sorption (–30 to –11 kJ mol⁻¹); delocalized charge and long alkyl groups increase diffusivity by up to three orders of magnitude. We also introduce an analytical equation to predict specific heat capacities from the molecular composition within 10% of measurements, resolving long-standing discrepancies in MD simulations and identifying the molecular weight per atom (Mw/N) as a governing parameter. These insights explain why many ILs excel in one property but fall short in others, and provide transferable, quantitative rules to balance trade-offs across applications. The framework enables high-throughput, interpretable discovery of ILs and hybrid phases for industrial CO2 capture, energy storage, and closed-loop life-support systems.